The invention relates to reactors, and particularly to reactors which contain a large number of vertical tubes which are adapted to be filled with a particulate material such as a catalyst through which a reactant fluid is passed. An example of such a reactor would be one in which an exothermic polymerization or oligomerization reaction takes place as a reactant fluid passes downwardly through the catalyst particles which are present in the plurality of tubes. For example, the reaction could be the conversion of an olefinic monomer into an olefinic dimer, trimer or tetramer. More specifically, the reaction could convert ethylene gas into butenes, hexenes or octenes. In a particular commercially available reactor which it is intended be modified, the upper and lower ends of the catalyst containing tubes are mounted in tube sheets which cooperate with the upper and lower ends of the reactor vessel to form an upper inlet chamber for receiving an incoming charge to be reacted with the catalyst and a lower outlet chamber for receiving the reaction product. The portion of the vessel which surrounds the tubes is filled with a heat exchange fluid. Typically, where the reaction is exothermic, the heat exchange fluid would be cooling water circulated at a rate sufficient to keep the catalyst in the tubes at a uniform desired temperature which is sufficiently high to promote good conversion but not so high as to significantly reduce the life of the catalyst or to produce undesired side reactions.
In the existing prior art vessels which are intended to be modified by the present invention, the fluid charge to be reacted with the catalyst is fed downwardly through all the tubes in the reactor, typically about 180 tubes, in a single pass. The restriction of flow to a downward direction avoids the possibility of catalyst attrition which could result if the catalyst were fluidized by upward flow through it. However, to achieve the contact residence time which is usually required between the reactant fluid charge and the catalyst in a vessel, the flow velocity of the fluid must be only about half as much in a single pass arrangement through a large number of tubes as it would be if it were possible to use a dual pass arrangement with flow through half as many tubes in each pass. Where several vessels are utilized in a single operation, it would be possible to increase the flow velocity by moving the fluid in series through the various vessels. For example, where the tubes of three vessels are connected in series, the reactant fluid, for a given contact time, could be moved at triple the velocity that would be possible if the tubes of the vessels were connected in parallel. Obviously, the increased velocity would increase the pressure drop through the tubes and there are some relatively fragile catalysts that could not tolerate a substantial velocity increase. However, where a catalyst can tolerate an increase in flow velocity and pressure drop, the increase in velocity can be extremely beneficial since it will cause the heat transfer coefficient to be increased quite substantially. Simultaneously, the mass transfer resistance between the bulk fluid and the catalyst will be reduced. Also, the catalyst life will be prolonged due to the elimination of "hot spots" which can develop at low velocities because of relatively poor heat transfer between the fluid and the catalyst. Another problem with the low velocity arrangements is that such a low pressure drop is present that very small variances in the quantity of catalyst present in the several tubes can result in nonuniform flow distribution. For example, if the head of catalyst is higher in one tube than another due to variations in packing, the tube with the greater head will present a greater resistance to flow and thus will see very little flow as compared to a tube with a lesser head of catalyst. Thus, some catalyst will be underutilized and other catalyst will be overutilized. The problem tends to be eliminated as overall pressure drop increases, since variations in pressure drop due to changes in catalyst head from tube to tube would become relatively insignificant compared to the overall pressure drop. For example, increasing the flow velocity by 400% results in an approximately 1600% increase in pressure drop.
Prior art patents which relate to reactors with multiple beds of contact material include U.S. Pat. Nos. 2,835,560, 3,424,553, 4,225,562, 4,308,234 and 4,461,745. U.K. Patent Application No. 2,120,119-A is also of interest in that it provides serial horizontal flow between a plurality of compartments with the addition of fresh gas between compartments. U.S. Pat. No. 4,461,745 shows a basic shell and tube reactor structure of the general construction which it is desired to modify. However, the disclosed design contemplates upward flow through the catalyst, a condition which could not be tolerated in the instant situation.